CURRENT TRANSFORMER ACCURACY

The ANSI ct accuracy class is determined by a letter designation and a secondary terminal voltage rating. These effectively describe the steady-state performance. (See IEEE Std C57.13-1993, 6.4.1.)

The secondary terminal voltage rating is the ct secondary voltage that the ct will deliver when it is connected to a standard secondary burden, at 20 times rated secondary current, without exceeding a 10% ratio error.

Furthermore, the ratio correction shall be limited to 10% at any current from 1 to 20 times rated secondary current at the standard burden or any lower standard burden. The voltage rating given applies to the full winding ratio only.

If a tap is utilized on a multi-ratio ct, the voltage capability is directly proportional to the ratio between the tap value being used and the full winding capability, provided the windings are fully distributed around the core. This is usually the case with cts made after 1978, but not necessarily with cts made before that date.

For example, ct accuracy class C100 means that the ratio error will not exceed 10% at any current from 1 to 20 times rated secondary current with a standard 1.0 W burden (1.0 W times 20 times rated secondary current equals 100 V).

Almost all of the cts used for protective relay applications are covered by the C or K classification. This includes bushing cts with uniformly distributed windings and other cts with minimal core leakage flux.

NOTE - IEEE standard C values and standard burdens are listed in annex B.

The letter designation codes are as follows:

C indicates that the leakage ßux is negligible and the excitation characteristic can be used directly to determine performance. The ct ratio error can thus be calculated. It is assumed that the burden and excitation currents are in phase and that the secondary winding is distributed uniformly. (See 8.1.10 of IEEE Std C57.13-1993 for further detail.)

K is the same as the C rating, but the knee-point voltage must be at least 70% of the secondary terminal voltage rating.

T indicates that ratio error must be determined by test. The T class ct has an appreciable core ßux leakage effect and contributes to appreciable ratio error.

H, L are old ANSI classiÞcations. There were two accuracy classes recognized-2.5% and 10%. Cts were specified in the following mannerÑ10 L 200, 2.5 H 400, etc. The first number indicated the accuracy class
and the last number indicated the secondary voltage class. L cts were rated at the specified burden and at 20 times normal current.

H cts were rated at any combination of burden from 5 times to 20 times the normal current. These ratings are applicable only to old cts mostly manufactured before 1954.